sql - 如何减去特定条件为真的产品数量

Question

    Date     Visitorid  Visitnumber  Condition A 
    1/1/2017    123        2             OF
    1/1/2017    123        3             OF
    1/1/2017    123        30            ABC
    1/2/2017    123        34            OP
    1/1/2017    124        1             OF
    1/2/2017    124        2             OP

我想减去首先访问'OF'然后访问'OP'的访问次数visitorId（但它应该在不同的日期）并且还想将上述结果分组为

 Count of visitor_ids where minus (of visit number is 1)
 Count of visitor_ids where minus (of visit number is 2)

等等